Replies

1. 
   Piffin | Dec 01, 2005 01:57am | #1

   Save me a headache and say what you are trying to do.

   The title kind of hints that you have some sort of octagonal roof to frame. If so, how large? What pitch? What layout?

    

    

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1. 
   lokiloki1 | Dec 01, 2005 02:10am | #2

   The pitch is a seven. Basically what I am trying to do is figure out how to step out a 7/13  rafter.  Obviously you set your dogs at 7 and 13 and step from there, but do you take 3/8"s instead of 3/4"s because it is halfway between a hip and a common? I am using standard 2x stock.

1. 
   Piffin | Dec 01, 2005 07:29am | #3

   It looks like what you are talking about is the hip drop and its relationship to HAP, or height above plate.on a typical hip it is not always 3/4" but varies with the pitch of the roof, though 3/4" is a fairly close assumption that will work for most common pitches. But I am still unsure if you are talking abouyt doing an octogonal roof and what the layout is.how long are the rafters? are they to be laid out at 16" OC?Meanwhile, try the advanced searcch button to look at other threads where HAP has been discussed. 

    

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2. 
   olle | Dec 01, 2005 03:37pm | #4

   I think you are trying to get the ridge deduction which is half the diagonal distance of the ridge thickness
   , measured along a 22 1/2 deg. angle. I would rarely step a rafter with a framing square . I would do the math. Check the threads for joe bartok's angle calculator and go from there. In order to tell you how much to deduct we would need to see exactly how things come togeather at the ridge, there several ways to do this

1. 
   JoeBartok | Dec 02, 2005 09:23pm | #5

   Ooops!, didn't see this post earlier. I'm not certain what your are trying to calc (judging from the title of the thread and the 7/13, an octagon?). I always nuke the angles first with a Hip-Valley Framing Calculator.

   The dimensions returned by this calculator are to the theoretical "framing points", the intercepts of centerlines of the Hips/Valleys, eaves, commons, etc. Heel height, soffit overhang, ridgeboards ... are not automatically factored into the calculations. But there are links to sketches and formulas which may help you.

   Btw, assuming your Hip rafters are 1½" stock, for 7/12 commons intercepting at a 135° angle in plan (a regular octagon), the Hip drop will be 0.16742" or approx. 5/32".

   Olle has a good point with regard to how things come together at the ridge. Are the commons meeting one another, or the hips? Any chance you can post a sketch?

   Joe Bartok

   Edited 12/2/2005 1:25 pm ET by JoeBartok

   Edited 12/2/2005 1:26 pm ET by JoeBartok

   Edited 12/2/2005 1:52 pm ET by JoeBartok

   Edited 12/2/2005 1:54 pm ET by JoeBartok

   Edited 12/2/2005 1:57 pm ET by JoeBartok

1. 
   lokiloki1 | Dec 03, 2005 02:22am | #6

   Thanks for all the advice folks.  I'm sorry I wasn't able to explain myself better.  The turret really isn't an octogon, it is a bay. The framing is 16 oc, and the stock is 2x.

   There will be a ridge in this situation, that lays on to the main roof. The king will butt the ridge, as will two commons on either side.  The two hips will be flown next with the plumb cuts at the top nestling in the 90 created by the first three rafters.  Finally the the 13 rafters will be flown and these will rest in in the 45's creatated by the hip, common, king intersection.

   When I have framed hip roofs in the past I have never had any problems with taking 3/4 out of the plumb cut in the birds mouth to suck the hip into plane. Obviosly the hip rafter comes in at a 45.  I take the 3/4 out because the stock is 2x and 3/4 is half the thickness.  Since the 13 rafter comes in at 22.5 or half of 45 I'm going to try taking 3/8s instead of 3/4 out of the heel and see what happens.  I'll let you know if it works out.

2. 
   JoeBartok | Dec 03, 2005 06:40pm | #7

   I sketched up a couple of Octagon Plan Rafter Arrangements last night ... and it turns out none of them quite apply. What you are framing would be most similar to half of the octagon shown in last image (except I'm showing all commons and no hips; I didn't have time to finish the drawing).

   "Since the 13 rafter comes in at 22.5 or half of 45 I'm going to try taking 3/8s instead of 3/4 out of the heel and see what happens."

   The geometry doesn't work that way and I still think your hip drop will be 5/32" not 3/8". For example, if an octagon were to be framed as per the second drawing, for the principal commons to "plane in" their lengths would have to be calculated as if the Hips were backed, from a point .16742" or 5/32" (the "Hip drop") above the center.Joe Bartok

3. 
   JoeBartok | Dec 03, 2005 07:02pm | #8

   Check out this Hip Drop animation. Don't worry about the trig or the unit dimension; this can be done with a compass and straightedge. Scale out a drawing beginning with a 135° plan angle with the centerline of the Hip bisecting the plan angle and follow the geometry in the animation.

   Joe Bartok

   Edited 12/3/2005 11:04 am ET by JoeBartok